n, k, x, y = map(int, input().split())
A = list(map(int, input().split()))
kouho = []
for a in A:
    kouho.append((a - 1 + k - 1) // k)
kouho.sort()
lim = y // x
behomara = 0
behoimi = 0
if n - lim - 1 >= 0:
    behomara = kouho[n - lim - 1]
for i in range(max(n - lim, 0), n):
    behoimi += kouho[i] - behomara
print(x * behoimi + y * behomara)